Fully integrated all-CMOS AM transmitter with automatic antenna tuning

ABSTRACT

A monolithic AM transmitter is disclosed. An external antenna forms part of a resonance network so that the antenna resonance point is automatically tuned to the transmit frequency. This provides flexibility with no added cost to the transmitter.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on the provisional application serialno. 60/046,128, filed May 9, 1997, entitled Fully Integrated All-CMOS AMTransmitter With Automatic Antenna Tuning, by J. Scott Elder, Joseph T.Yestrebsky, and Mohammed D. Islam.

FIELD OF THE INVENTION

[0002] This invention relates to transmitters and, in particular, to anintegrated transmitter with automatic antenna tuning.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]FIG. 1 is a block diagram illustrating the functional units in atransmitter in accordance with one embodiment of the invention.

[0004]FIG. 2 identifies the pins of an integrated circuit incorporatingthe transmitter.

INTRODUCTION

[0005]FIG. 1 illustrates the basic structure and elements of oneembodiment of the invention, a Fully Integrated All-CMOS AM Transmitterwith Automatic Antenna Tuning. All elements are constructed usingComplementary Metal Oxide Semiconductor (CMOS) technology. CMOStechnology also embodies other bipolar type elements like diodes,transistors, and others. Where appropriate, these device types are used.The elements of the invention are as follows, with associated referencenumber: Functional Element Reference No. a. Reference Oscillator 1 b.Phase Detector 2 c. Loop Filter 3 d. Filter Tune 3a e. Track and Hold 4f. Differential Oscillator 5 g. Prescaler 5a h. Div_M 6 i. VaractorCharge Pump 7a j. Charge Pump 7b k. Bandgap Reference 8

[0006] By fully integrated, we mean that all of these functions in theirentirety have been simultaneously incorporated onto a singlesemiconductor die (integrated circuit or IC). This we believe is anextension of the present state-of-the-art. Aside from power supplydecoupling capacitors, no external components are required except anantenna (L1, L2), and antenna resonating capacitor (C1). Additionalaspects of the invention (to be detailed subsequently) further extendthe art by reducing overall transmitter system complexity, cost, whileimproving transmitter performance.

[0007] Special Charge Pump circuitry is included within the invention toallow the invention to function in applications which use a wide rangeof supply voltages. This makes the invention quite general in nature,and lowers overall radio system costs. To protect nFET devices fromhot-electron effects in applications with large external supplyvoltages, nFETs are cascaded to guarantee nFET drain-to-source voltageis clamped to acceptable levels.

DETAILED DESCRIPTION OF INVENTION

[0008] Reference Oscillator 1 generates a precise timing waveform basedon a timing element or timing signal applied externally at theextremities of the invention.

[0009] Phase Detector 2, Loop Filter 3, Filter Tune 3 a, DifferentialOscillator 4, and Div_M5 collectively form a phase-lock-loop (PLL) wherethe frequency of the Differential Oscillator is locked to that of theReference Oscillator.

[0010] Filter Tune 3 a itself is a phase-lock-loop tuned to theReference Oscillator. Filter Tune generates a bias current Ibias for theLoop Filter 3. This approach stabilizes the Loop Filter performancecharacteristics against semiconductor process and temperaturevariations.

[0011] The Differential Oscillator serves as the source of the transmitfrequency. It is important to note that this oscillator is within thePLL and is in actuality a voltage controlled oscillator (VCO).Oscillator tuning is accomplished by means of a differential structureof diffusion type p-n diodes within the Differential Oscillatorfunction. We call these varactor diodes in the invention. Externaltuning diodes connected in a differential manner will also work but comewith added system cost.

[0012] The Differential Oscillator provides a differential output into a(differential) antenna L1, L2. The antenna is coarsely tuned bycapacitor C1, and fine tuned to the Reference Oscillator frequency viathe varactor diode(s). L1, L2, and C1 are externally applied to theinvention. For less demanding applications, where harmonic distortion isless important, a Differential Oscillator may not be necessary. Analternative embodiment is a single-ended oscillator, which reduces ICpin count.

[0013] Addition of resistor Rbias to the invention sets the bias currentin the Differential Oscillator, which in turn sets the transmit powerlevel. For fixed transmit power applications, this bias may be fixed onthe semiconductor device.

[0014] Output from the Differential Oscillator is divided down byfunction Div_M 6, which produces a frequency near the ReferenceOscillator frequency, within the locking and tracking range of the PLL.A Charge Pump 7 provides voltage multiplication, which allows theinvention to operate with supply voltages below 3V. An externallyapplied pumping capacitor is required with the Charge Pump. This supplyvoltage generation technique can either be open loop controlled orclosed loop controlled. In a closed loop application, the charge pumpoperation ceases when the external supply voltage is sufficient forproper operation of the invention. In an open loop application, thevoltage settles to an integer multiple of the input voltage source. Theinvention design is capable of operating with an external supply voltagegreater than 12V.

[0015] Track and Hold 4 provides the means to hold the Loop Filter(frequency) control voltage during transmit ‘spaces’, when thedifferential Oscillator is disabled. This function requires anexternally applied hold capacitor Oh. Alternative embodiment is a Trackand Hold function where the hold capacitance is integrated (e.g., a DACapproach)

[0016] A Bandgap Reference circuit provides temperature and supplyvoltage compensated bias voltages and currents for critical nodes withinthe invention.

[0017] The invention allows for packaging in a 10 pin package. Inapplications where the charge pump is not required (i.e. adequate supplyvoltage exists for proper device operation), embodiments with fewer pinsare possible.

[0018] Numerous physical embodiments exist with this invention,depending on the intended applications and system requirements. The fullembodiment is illustrated in FIG. 2, along with those signals which arerequired on the invention, and those which depend on the application,system requirements, and level of integration of the invention. By levelof integration of the invention, we mean that one need not integrate theCharge Pump 7 b, for example, but provide the function external to theinvention. This removes the need to integrate the Charge Pump onto theinvention, which may be more desirable than the more highly integratedinvention.

[0019] Having said this, FIG. 2 illustrates that the minimum embodimentis a 5 pin IC package. Other package variations of 6, 8, 10, and 14 maybe appropriate for the invention depending on the integration level ofthe invention.

[0020] Operation of Invention

[0021] System Operation—Introduction

[0022] The invention, Fully Integrated All-CMOS AM Transmitter withAutomatic Antenna Tuning, converts a input (logic level) data sequenceonto an amplitude modulated (AM) RF carrier. The transmit antenna formspart of the resonant network of the Differential (RF) Oscillator on theinvention. This Differential Oscillator is enclosed within a closed-loopcontrol system (a PLL). Consequently, the antenna resonance point isautomatically tuned to the transmit frequency, which is an integermultiple of the Reference Oscillator on the invention.

[0023] Two Charge Pumps and appropriate control functions are includedon the invention to (1) provide sufficient bias voltage for varactordiodes, and (2) allow the invention to function properly for externallyapplied bias supplies below 3V.

[0024] Reference Oscillator 1

[0025] A Reference Oscillator 1 develops the precision timing signalwhich is used by the phase-lock-loop to set the transmit frequency. TheReference Oscillator design is of the Colpitts variety, and is connectedto a timing device external to the integrated circuit (IC). Typicaltiming devices are ceramic resonators, crystals, or (tuned)inductor-capacitor tank circuits. Phase lag capacitors generallyassociated with ceramic resonators are integrated onto the IC to lowercost.

[0026] The output from the oscillator is dc-coupled into an single-endedamplifier which provides differential outputs. The amplifier provides nodc gain. This eliminates any dc offset correction requirements. Theamplifier output is differentially coupled into a comparator whichlimits the output waveform to logic level swings. The output stage ofthe comparator is ac current balanced to eliminate any unnecessary powersupply noise. The Reference Oscillator design requires only a single ICpin for connection of the precision timing element or signal.

[0027] Finally, the Reference Oscillator design supports the applicationof an external (precision) timing signal rather than a timing element.The signal is applied at the same point on the invention as the timingelement would be connected to.

[0028] Phase Detector 2

[0029] Phase Detector 2 provides a measure of the frequency differencebetween the output of the Reference Oscillator and the DifferentialOscillator frequency divided by scale factor M (within the Div_Mdivision function). The Phase Detector needs only to discriminatefrequency differences; phase error in this phase-lock-loop (PLL) doesnot affect performance of the invention.

[0030] A digital phase-frequency detector is preferred for two reasons.Firstly, the circuit output is differential in nature. Secondly, whenthe PLL finally acquires frequency and phase lock, the phase-frequencydetector output is a series of very narrow pulses in the time domain.Such a waveform is much easier to filter with an integrated Loop Filterthan a phase detector that exhibits quadrature phase lock.

[0031] Loop Filter 3 and Filter Tune 3 a

[0032] The output waveform of the Phase Detector contains both adc andan ac term; the dc term represents the frequency error between theReference Oscillator output and the Differential Oscillator divided byscale factor M. Only the dc term is of value; the ac term is removed bythe Loop Filter.

[0033] The Loop Filter is a gm-c type, for at least three reasons.Firstly, the variance in gm-c filter characteristics is almostcompletely related to the gm variances, since integrated capacitors arewell behaved. Secondly, a fairly high order filter may be constructedusing only modest amounts of integrated capacitance, which reduces thecost of the invention. Thirdly such filters are easily ‘tuned’ tostabilize their characteristics against semiconductor process andtemperature variations. Such tuning is provided in the invention from atuning PLL Filter Tune 3 a ‘slaved’ off the Reference Oscillator.

[0034] The function Filter Tune 3 a is a PLL which is referenced to theReference Oscillator. The PLL controls the bias current for acurrent-controlled-oscillator (ICO) within Filter Tune. The PLL adjuststhe bias current to force the ICO frequency to equal the ReferenceOscillator frequency. This may be accomplished directly or indirectlythrough the use of frequency prescalers (or frequency dividers). Theresultant bias current is mirrored to provide the bias current for LoopFilter.

[0035] Track and Hold 4

[0036] During transmission of ‘spaces’ the Differential Oscillator isdisabled. This ‘breaks’ the PLL closed4oop frequency control system. ATrack and Hold 4 is used to hold the PLL state just prior to disablingthe Differential Oscillator, signifying a ‘space’ transmission. This isnecessary for at least two reasons, (1) so that the PLL does not have torequire frequency lock for each ‘mark’ transmission, and (2) to assurethat the Differential Oscillator is not ‘chirped’, or swept in frequencyas the PLL settles out during a ‘mark’ transmission.

[0037] The Data input to the Track and Hold is the signal which controlsTrack vs. Hold operation. During ‘space’ transmissions, the function ispaced in the Hold mode.

[0038] Some applications may be better served (lower cost for theinvention) by the use of an external hold capacitor (Ch) for the Trackand Hold function, as shown in FIG. 1. However there are techniquesavailable to integrate the effective hold capacitance onto theinvention, with additional cost, and perhaps a modest compromise in the‘droop’ of the hold voltage. For example, to support very low datarates, an external hold capacitor is preferred, but as the operationaldata rate increases, the impact of integrating this capacitor is lessdeleterious to cost and performance. So an alternative embodiment is theintegration of Ch, which reduces the IC pin count by one.

[0039] Differential Oscillator 5

[0040] The Differential Oscillator is a voltage controlled oscillator(VCO) based on two cross-coupled Field Effect Transistors (FET's) wherethe antenna serves as the load impedance for the FET's. Thecross-coupled FET structure provides the feedback for oscillatorybehavior. The antenna is applied externally to the invention,differentially connected to the external bias supply, shown in FIG. 1 asL1 and L2. The antenna is brought coarsely in the range of the propertransmit frequency by connecting an external capacitor C 1 across theexternal (differential) antenna as FIG. 1 illustrates. This sets theDifferential Oscillator's nominal frequency of oscillation. This coarsecapacitor can also be integrated on the same silicon further reducingexternal component requirements. By integrating the coarse capacitanceon chip, the antenna design requirements are further restricted.

[0041] The Differential Oscillator frequency is adjustable via a dccontrol signal VAFC, which is the filtered output of the Phase Detector.Two reverse-biased p-n diffused-junctions are connected differentiallyacross the cross-coupled FET's of the Differential Oscillator. As the dclevel of VAFC varies, the capacitance of the reverse-biased p-njunctions varies inversely. This capacitance forms part of the resonanttank which sets the frequency of oscillation. Thus this differentialvaractor diode approach is the method used to modify the frequency ofthe oscillator. An alternative varactor approach is to use the CVcharacteristics available in an MOS diode.

[0042] The bias current for the Differential Oscillator is set viaaddition of resistor Rbias externally to the invention. A closed-loopvoltage follower on the invention regulates a voltage based on theinternal Bandgap Reference 8 on the invention. Rbias sets the currentbased on its value and the regulated voltage; this current is mirroredto the Differential Oscillator as the bias current. An alternative toproviding Rbias externally is to integrate this onto the invention as afixed, unmodifiable current. This will result in a fixed, unmodifiabletransmit power level. If the on chip Rbias resistor is built as adigital to analog converter, the transmit power level can be varied bytransmitting to the invention (via a separate input pin) a data patternto set the desired transmit power level.

[0043] Input Data is used to electronically steer the bias current tothe Differential Oscillator. During ‘mark’, transmissions, the biascurrent is connected to the Differential Oscillator. The DifferentialOscillator then begins to oscillate, transmitting a RF carder during the‘mark’, interval. Alternatively, during a ‘space’, the bias current isdisconnected from the Differential Oscillator, and so oscillationceases, transmitting the equivalent of a ‘space’.

[0044] Special back-to-back trapped drain Field Effect Transistor (FET)structures are employed at critical high frequency nodes of theDifferential Oscillator to minimize capacitance and improve bandwidth.The preferred embodiment is circular drain structures, although otherstructures (octagonal, hexagonal, etc.) are acceptable. This structurehas important ramifications, since the result is a very small capacitiveparasitic at the drain node of the FET and a larger capacitance at thesource node of the FET. This ‘breaking-apart’ of the normally equaldrain and source parasitic capacitances actually further improves thegain-bandwidth (GBW) of the Differential Oscillator beyond just theimprovement anticipated by a lowering of the drain capacitance. Saidanother way, the extra source capacitance actually improves GBW.

[0045] The invention employs a Differential Oscillator for a goodreason, namely that such a structure driving a differential antennaresults in reduced harmonic distortion of the transmitted signal. Thisis important in some critical applications, but is less important inother applications. Thus an alternative embodiment is the use of anon-differential oscillator where application requirements permit. Thisreduces the cost of the invention, as well as its pin count. It isanticipated that a differential scheme will be necessary to meet mostinternational regulatory requirements similar to the United States FCCand Part 15 of the Code of Federal Regulations.

[0046] Prescaler 5 a

[0047] The Differential Oscillator operates at a very high frequency,beyond the operating range of conventional CMOS logic dividers. So thisfrequency must be divided down by a high speed circuit before furtherdivision by conventional logic can take place. This circuit is thePrescaler 5 a. The Prescaler is composed of a concanternation of highspeed cross-coupled FET divide-by-two circuits.

[0048] The Prescaler is biased using a current which is tuned to theReference Oscillator 1. This guarantees that the Prescaler bandwidth istemperature and process independent.

[0049] Special back-to-back trapped drain Field Effect Transistor (FET)structures are employed at critical high frequency nodes of thePrescaler to minimize capacitance and improve bandwidth.

[0050] Div M_(—6)

[0051] Once the Differential Oscillator frequency is prescaled to anappropriate lower frequency, conventional CMOS logic dividers are usedto further divide the frequency down to approximately the ReferenceOscillator frequency. This is accomplished by division function Div_M 6.

[0052] Varactor Charge Pump 7 a

[0053] The reverse-biased p-n junction capacitors (i.e., varactors) inthe Differential Oscillator require a large bias potential for properoperation. On the other hand, the varactors require very little biascurrent. The need for extra circuitry to construct the varactor biasvoltage depends on the magnitude of the bias supply externally suppliedto the invention.

[0054] For applications where the external bias supply is in thevicinity of 12V, extra bias generation circuitry is not required.However, for lower external bias voltages, a circuit is required tosynthesize a larger varactor bias voltage from the smaller external biasvoltage. This function is preferentially accomplished by a voltagemultiplying charge pump, since the load current is very small. Such acircuit in CMOS is easily integrated onto the invention without the needfor additional components external to the invention. Thus no additionalpins are required to provide this function within the invention.

[0055] Charge Pump 7 b

[0056] Applications for this invention often demand operation withexternal bias supply voltages ranging from 12V down to below 3V.However, operation at bias voltages much below about 5V in CMOS isdifficult without special processing (to lower FET threshold voltages),and the use of sub-micron or deep-sub-micron technology. To meet thisrequirement using inexpensive technology, a voltage multiplying ChargePump 7 b is included in the invention. The purpose of the Charge Pump isto synthesize supply voltages of sufficient magnitude (generally greaterthan approximately 4V) from externally applied voltages lower than thisvalue. The Charge Pump output is then used instead of the external biasvoltage to power-up the other elements of the invention which requirethis higher voltage.

[0057] The Charge Pump 7 b is different from the Varactor Charge Pump 7a. Firstly, the Charge Pump must supply much larger output currents thanthe Varactor Charge Pump. Secondly, the varactor Charge Pump is requiredfor the invention to operate except in applications where the externalsupply voltage is approximately 12V. The Charge Pump however is onlyrequired for applications where the external supply voltage is belowabout 5V. Also, the Varactor Charge Pump requires no external pins onthe IC package. The Charge Pump requires several package pins, for apumping or ‘flying’ capacitor (Cfly), and for a filtering capacitor (COon the synthesized output voltage.

[0058] Thus it is likely that there are several physical embodiments ofthe invention. It is perhaps most cost effective to include an VaractorCharge Pump on any physical embodiment of the invention since this costsno package pins and only modest CMOS die area. However the inclusion ofthe Charge Pump is expensive due both to die area and required packagepins, and thus may not be included on embodiments intended forapplications with greater than about 5V external supply voltages. Onefurther alternative is not to include the Charge Pump on the physicalconstruct of the invention, but to provide this function externally, forsub-5V supply applications.

[0059] Bandgap Reference 8

[0060] A Bandgap Reference circuit is included on the invention. Thepurpose of this function is to generate reference voltages which aretemperature and supply voltage stable. Stable reference voltages arerequired to bias various critical nodes within the invention.

[0061] Alternative Embodiments and Enhancements

[0062] By implementing the invention in CMOS, it is economical toinclude encoder functions on the same IC since most of the applicationsfor this type of radio transmitter include a encoder before thetransmitter. The encoder can be either fixed or variable (i.e.,programmable, like a microprocessor).

[0063] Other embodiments include the addition of a shutdown mode forlow-power duty-cycling of the invention.

[0064] An encoder is a circuit function that generates a on/off datastream which ultimately turns on/off a transmitter function. Thetransmitter is generally either infrared based or RF based orelectromagnetic field based. Obviously, other embodiments of atransmitter are possible.

[0065] The encoder on/off data stream can either be fixed or variable.An example of a fixed data stream would be one permanently programmed inan encoder circuit with either switches or jumper connections. Anexample of a variable encoder data stream is one similar to that used inrolling code encoder systems. Rolling code schemes are generally foundin high security applications where the user is trying to evade captureof a valid encoder data stream for later broadcast to a receivingdecoder. Generally, these rolling code encoder schemes embody erasableand reprogrammable memory circuits. In that this invention now allowsfull integration of at least one embodiment of a transmitter function,all of the encoder circuit schemes can be integrated on to one singlesilicon integrated circuit at a very low cost.

[0066] In that this invention is built on a CMOS process technology, amicroprocessor or similar computing circuit element can be combined onthe same silicon integrated circuit with the invention since modem daylow-cost microprocessors are almost always constructed on a similarprocess.

[0067] The invention has certain performance limits today that areexpected to change in the future. One such limit is the achievable upperoperating frequency limit. One skilled in the art will readily recognizethat the invention disclosed here can continue to be scaled as CMOStechnology is scaled to continue applying the invention at higher andhigher operating frequencies.

[0068] The United States agency that regulates unlicensed radio or RFtransmitters is the FCC. The FCC has allocated certain frequency bandsand emission types for which this type of device is intended. The Worldhas several regulating bodies which in a similar manner control thesesame issues. In some instances, the World's other regulatory agenciesare more restrictive in there operating rules. This invention isintended for operation under all of the World's various regulatoryagency provisions.

[0069] The primary application of this invention, but certainly not theonly application, is for automotive keyless entry, garage door openers,home keyless entry, security systems, remote door bell ringers.Obviously, there are numerous other applications that combine a simpletransmitter with a digital data stream generation circuit.

[0070] This invention's uniqueness is further exemplified by theobservation that a complete RF radio transmitting system can be builtwith no other external RF components except an antenna.

[0071] While the preferred embodiment of the invention is an AM typetransmitter, one may also build an FM transmitter with virtually thesame circuit techniques described herein. The modifications that arenecessary to build FM transmission entail changing the PLL divisionratio or reference frequency by the requisite frequency deviation forthe system. This will force the control system to tune the antennabetween any of a number of frequencies determined by the PLL feedbackdivisor or reference frequency values. In the limit, these valuesapproach infinity. A simpler FM modification is also possible with theinvention, namely varying the varactor diode bias directly. This is amore traditional technique of FM modulation although not currently doneon CMOS silicon at the frequencies made possible by this invention.

[0072] As a result of the economy of scale associated with CMOS processtechnology, an extension of this invention is to build severaltransmitters on the same silicon die. This will allow one to buildlow-cost multi-channel systems which are either AM, FM, or otherwise. Amultichannel system enhances the achievable data rate in much the samemanner that increasing a digital systems data bus or address bus width.

What is claimed is:
 1. A transmitter comprising: an oscillator forming part of a phase-lock loop (PLL); a modulator, said oscillator and said modulator being formed on a single monolithic chip; and an antenna, said antenna forming part of a resonant network of the oscillartor, a resonance point of said antenna being automatically tuned to a transmit frequency of said transmitter. 